This invention relates to improvements in large scale underwater risers, and particularly is such as to provide a buoyancy system for large scale underwater risers which may be effective in deep waters, e.g., ocean waters of a depth of 3,000 meters or more.
Deep underwater drilling has become a requirement in order to tap sources of hydrocarbons from sites well below 1,000 meters or more underwater. In such drilling, a long drilling riser conduit extends between the site at the ocean floor to the vessel or floating platform. Such riser normally comprises a string of units (known as joints), the individual units being connected by means of flanges with one another.
One of the problems engendered in deep sea drilling using riser conduits is the problem of locating and maintenance of the riser with respect to the platform or vessel, particularly where the surface vessel or platform may be subjected to considerable movement both horizontally and vertically due to current, wave and wind action. Such problems, of course, may subject the risers to excessive axial and buckling stresses.
Generally speaking, a principal requirement for stability of the riser--i.e., immunity to buckling or other stress failures, etc.,--is that the riser must be maintained effectively in tension over its entire length. More specifically, the effective tension in a riser must be considered to be the pipe wall tension diminished by the effects of pressure differential across the pipe wall, seawater pressure gradient, and so on.
Another problem which is encountered at sea, particularly in deep water conditions, is that occasionally the buoyancy of a riser system may be required to be adjusted, sometimes very rapidly.
Thus, while in the past the riser string has been kept under tension by such means as pulling on the upper end of the riser, either using counterweights or automatic tensioning equipment located on the vessel, the continuing search for hydrocarbons in deeper ocean environments has made these proposals, on their own, incapable of handling greater depths.
Of late, accordingly, it has been proposed to provide buoyancy devices for risers which would be capable of attaining the required buoyancy capabilities at greater depths, so as to properly maintain the risers. One such means has been the use of syntactic foam; and floatation air cans have also been proposed as buoyancy devices for deep sea risers.
A well known detriment of syntactic foam, however, is that it loses its buoyancy capacity due to absorption of water or compaction of the syntactic material, especially at increased depths. Thus, acceptance testing--i.e., testing prior to actual use--is normally a requirement for these foams, primarily to determine the buoyancy loss due to the ingress of water, so that allowances can be made for such losses. Further, any damage to the skin of such foams may materially accelerate the diminishing buoyancy capacity. Visual inspection does not normally enable a determination to be made as to the relative capacity of the foam, and it therefore may require a check of the air weight of the foam in order to determine its relative floatation or buoyancy capacity.
Moreover, while syntactic foam does provide passive buoyancy, such that its buoyancy level remains relatively constant if buoyancy losses are discounted, its ultimate depth capability is limited. Still further, in an emergency situation, (or indeed a planned dis-connect situation) where it is necessary to rapidly reduce buoyancy of a riser in order to maintain stability of such as a pendulating riser string, it is very expensive to provide means to dump the syntactic foam and especially when it is considered that it is probably or practically impossible to recover the syntactic foam once it has been dumped.
There have also been several floatation air can designs proposed to provide riser string buoyancy for deep sea drilling.
According to one prior art proposal, as disclosed in RHODES et al, U.S. Pat. No. 3,017,934 dated Jan. 23, 1962, a riser is buoyantly supported by a plurality of buoyancy chambers or cans, the chambers or cans being of progressively greater buoyancy per unit length in the direction along the longitudinal axis of the member with increasing water depth. In accordance with one embodiment disclosed by RHODES et al, buoyancy cans are provided which are directed with their open bottoms towards the ocean floor, which cans may be filled from a supply of gas leading to the bottom most can, nearest the ocean floor. A gas conduit allows the gas to flow from a full buoyancy can to the can immediately next above it until all the cans or pods are filled by the gas, which is usually compressed air. Of course, no gas is applied to the next can until the preceding one has been filled.
A more recent proposal is advanced in WATKINS U.S. Pat. No. 3,858,401, dated Jan. 7, 1975, and assigned to Regan Offshore International, Inc. According to WATKINS, floatation for underwater well risers is provided by a plurality of open bottom, buoyancy gas-receiving chambers, which are mounted about the riser conduit. A gas conduit is provided by WATKINS for the delivery of a gas, such as compressed air, to each of the chambers. Gas is admitted to each chamber through an associated valve for each chamber, each of the valves having a floating valve member. Gas supply to a chamber is discontinued when the valve member closes the valve orifice on replacement of the water in the chamber, i.e., when the floating valve member is no longer supported by water. Thus when upper chambers are filled by the gas, and on closing of the valve associated with each chamber, the gas can flow into the next chambers below, instead of gas leaking from the bottoms of the upper chambers.
The proposal by WATKINS suggests embracing the riser by concentrically disposed, open ended chambers. While this system maximizes use of the space for air buoyancy, the system produces a significant pressure differential between the gas--usually air--and the surrounding water which must be accounted for in the structural design of each of the chambers. Furthermore, it is common practice to stack the risers prior to use, such as on the deck of the transport vessel or floating platform. Since the chambers concentrically surround each riser section or unit, the walls of the chamber must, therefore, exhibit the required strength. Thus, the chambers tend to be very heavy, thereby offsetting a significant percentage of the buoyancy gained.
Also, in order to allow for handling and storage, as the containers are attached to each riser section during such handling and storage, the chambers of the WATKINS systems are designed to present a smooth circular outer surface concentric to the axis of a riser. Such a smooth hydrodynamic surface is not desireable due to an increase of drag forces imposed by sub-surface currents and in waves, and the riser may be subject to vortex shedding vibration. In addition, the WATKINS system has certain difficulties due to the possible flexing of the riser conduit within the relatively rigid air chamber or container which surrounds it.
It will, of course, be apparent that a multiplicity of valves and the attendant piping can lead to malfunctioning of at least some of the valves, thereby possibly reducing the efficiency of the system.
The WATKINS patent indicates that the system can be used in drilling operations at up to depths of 6,000 feet (1,829 meters) below the water surface.